Burners for chemical analysis

ABSTRACT

A burner for chemical analysis such as flame spectroscopy, is provided with a burner plate having a flame aperture comprising a long slot extending through the upper and lower surfaces of the burner plate. The slot has a longitudinal profile taken at right angles to the length of the slot which is at least partly arcuate about an axis parallel to the length of the slot and tapers inwardly from the fuel inlet region adjacent the lower surface of the burner plate. In one embodiment the lower edge of the slot is defined by an arc of a circle at least 3 mm in radius. The specified profile minimizes turbulence in the slot region adjacent the lower surface of the burner plate and hence reduces the deposition of sample material on the burner plate.

United States Patent 1191 George May 14, 1974 BURNERS FOR CHEMICAL ANALYSIS [75] Inventor: Richard Alexander George, f w'bert Cambridge England Asszstant Examiner-F. L. Evans Attorney, Agent, or Firm-Frank R. Trifari [7 3] Assignee: Pye Limited, Cambridge, England 221 Filed: Jan. 30, 1973 1 ABSTRACT A burner for chemical analysis such as flame spectros- [211 Appl' 328034 copy, is provided with a burner plate having a flame aperture comprising a long slot extending through the [30] Foreign Application Priority Data upper and lower surfaces of the burner plate. The slot Feb. 2. 1972 Great Britain 4899/72 has a longitudinal Pmfile take" at right angles t0 the length of the slot which is at least partly arcuate about 52 us. 01 239/597, 239/601, 356/87, an axis Parallel to the length of the Slot and tapers 431/126 wardly from the fuel inlet region adjacent the lower 51 1m. (:1 B05b 1/00 Surface of the urne pl In one em odiment the [58] Field of Search 356/87, 187; 431/126; lower edge of the slot is defined y an are of e eirele 239/568, 592594, 596, 597, 601 at least 3 mm in radius. The specified profile mini- 2 mizes turbulence in the slot region adjacent the lower [56] References Ci d surface of the burner plate and hence reduces the de- UNITED STATES PATENTS position of sample material on the burner plate. 3,516,771 6/1970 Rendina 356/87 2 Claims, 4 Drawing Figures PATFNWIIII 14 I914 1 3.81 O 58 3 SHEET 1 0F 2 RADIATION SENSITIVE INTENSITY DEVICE METER 18 21 22 '23 21. g I

28'538? MONOCHROMATOR AMPLIFIER 12 19 NEBULISER CLOUD 5CHAMBER Y? 11 17 14 SAMPLE FUEL AI AIR Fig.1

PATENWMAY 14 974 saw 2 OF 2 1 BURNERS FOR CHEMICAL ANALYSIS This invention relates to burners. for chemical analysis apparatus of the kind in which a sample of material is burnt and the flame in which combustion takes place subjected to optical analysis.

An example of such apparatus is an atomic absorption spectrophotometer in which the function of the burner and flame is to produce free atoms of the sample to be determined. In order that the overall sensitivity of the instrument may be good and readings consistent, it is important that the production of atoms should be both efficient and regular.

The most commonly adopted form of burner for atomic absorption spectrophotometers includes a burner plate, generally of rectangular shape, which is relatively thick. The burner plate may either be of unitary construction or be assembled from two or more separate parts. A straight-sided slot through the burner plate functions as the flame aperture, and the burner plate is usually mounted upon a sample/fuel gas receiving chamber of suitable construction.

The plate-type burner construction is generally preferred to others because of its ability to provide a more laminar gas flow through the flame aperture. However, this type of burner is not completely satisfactory in use as turbulence can occur in the region of the slot adjacent the lower face of the burner plate causing fluctuations in the sample/fuel gas flow through the aperture and, when high concentration samples are being aspirated, deposition of the sample in this area, which may block the flame aperture.

It is an object of the present invention to provide an improved plate-type burner for chemical analysis apparatus which is less prone to turbulence in the region of the flame aperture adjacent the lower surface of the plate.

According to the invention there is provided a burner for chemical analysis apparatus including a flame aperture in the form of a longitudinal slot which extends through the upper and lower surfaces of a burner plate and has a profile in cross-section which tapers inwardly from the region adjacent the lower surface of the burner plate, the profile being at least in part arcuate about an axis parallel to the longitudinal axis of the slot.

Provision of the specified profile minimises turbulence in the region of the slot adjacent the lower surface of the'burner plate and hence reduces the deposition of sample material.

In a preferred embodiment the lower edges of the slot have a profile which is'defined by the arc of a circle of radius 3 mm or greater.

In order that the invention may be clearly understood and readily carried into effect an embodiment will now be described by way of example, with reference to the accompanying drawings, of which:

FIG. 1 is a diagram illustrating, in general form, an atomic absorption spectrophotometer,

FIG. 2 is a perspective view of a burner plate of a burner embodying the invention,

FIG. 3 is a cross-sectional view taken along line III- -III of FIG. 2, and

FIG. 4 is an outline of the view of FIG. 3 indicating the principal dimensions.

Reference will now be made to FIG. 1 which shows in general form an atomic absorption spectrophotometer including a nebuliser 10, a cloud chamber 11 and a burner 12. The nebuliser 10 is fed with compressed air from a cylinder 13 and with sample solution from a vessel 14. Fuel and auxiliary air are fed from cylinders 15 and 16 through a common line 17. Mixing of the fuel/air mixture and sample solution spray occurs in the cloud chamber 11, and the resulting mixture is fed to the burner 12 which produces an analytical flame 18 wherein evaporation of the sample solution solvent takes place and vaporisation of the sample occurs.

Radiation from a source 20 provides a beam of radiation having one or more characteristic spectral line(s). The beam of radiation emitted by the source 20 is focussed and passed through the flame 18 containing the vapour of the sample solution containing an unknown quantity of the. material under examination, into a monochromator 21. Certain spectral lines are absorbed by the vapour of the sample from the beam of radiation indicating the presence of a particular element in the vaporised solution if the beam of radiation is of a specific wavelength corresponding to the energy required to excite atoms which have been vaporised, from their ground state to the resonant energy level.

Adjustment of the monochromator 21 to the wavelength of the spectral lines which are to be absorbed gives a beam of radiation of narrow bandwidth about that wavelength. The resultant beam of radiation is directed onto a radiation sensitive device 22 which provides an output signal which is amplified by an amplifier 23 and applied to a meter 24 which provides an indication of the intensity of the radiation leaving the monochromator 21. The degree of absorption of radiation of a given wavelength may be measured by comparing the indication displayed by the meter 24 when unknown samples are introduced into the flame l8 and when no sample is introduced into the flame 18.

The construction of a burner 12 which embodies the invention will now be described with reference to FIGS. 2, 3 and 4. In FIG. 2, a burner plate generally indicated as 1 comprises two identical jaw portions 2 and 2' held together by means of screws 3 and 3'. The two jaws are spaced apart by spacers 4 and 4' whose inner ends 8 and 8 limit the extent of the longitudinal flame aperture slot 5 so formed. The burner plate 1 mounts upon acasting or fabricated duct 19 in FIG. 1, which serves to convey fuel gas and nebulised sample solution to the flame aperture 5. The analytical flame is established along the mouth of the aperture 5.

The edges 6 and 6 of the jaws 2 and 2 are radiused to minimise turbulence in the fuel gas/sample mixture fed under pressure to the flame aperture 5 and aid transfer of the nebulised fluid sample through the aperture 5. The absence of arectangular profile adjacent the lower surface 7 of the burner plate minimises the deposition of sample material in this region and hence the obstruction of the aperture 5 when a highly concentrated sample solution is being aspirated.

The radiused edges 6 and 6 ofthe flame aperture 5 extend for substantially the whole length of the flame aperture 5. The inner ends 8 and 8 of the spacers 4 and 4' are preferably cut to provide the aperture 5 with an inwardly tapering profile in the longitudinal direction also.

Referring to FIG. 3 of the drawings, a burner constructed in accordance with the invention to burn a mixture of air and acetylene had burner plate dimensions as follows:

I 6.35 mm s 0.61 mm r 12.7 mm

x 3.05 mm The length of flame aperture was 100 mm. A burner for nitrous oxide and acetylene, also constructed in accordance with the invention, had the following dimensions:

l=5.l mm s=0.56 mm r= 12.7 mm h= 12.45 mm x 2.03 mm The length of the flame aperture 5 was 50 mm.

Generally the widths of the aperture 5 will be within the range 0.35 mm to 0.76 mm and the thickness h of the plate within the range 4.5 mm to 26 mm. In order to provide conditions of laminar flow the following relationships should be observed.

x 1.5s and l 411/5 and to obtain a maximum improvement in burner performance the radius of the arcuate portion of the aperture profile should be greater than 3 mm. The dimension x may be less than or equal to, but should not be greater than, the dimension r. To provide the smoothest transition between the arcuate and the straightsided portions of the aperture 5, the circle of which the arc is part should be tangential to the straight-sided portion.

it will be appreciated that the present invention provides an extremely simple but effective burner for chemical analysis apparatus by reducing turbulence and sample deposition in the region of the flame aperture adjacent the lower face of the aperture of the burner plate. Use of a burner plate in accordance with the invention in an otherwise standard instrument has improved instrumental sensitivity by a factor of x2 and has given a similar improvement in instrumental precision, relative to the performance of the same instrument when using a burner plate having a straight-sided flame aperture.

Although the invention has been described by reference to a burner having a burner plate constructed from a pair of burner jaws separated by suitable spacers the invention is equally applicable to burner plates fabricated in other ways, for example machined from a single piece of material or cast. A burner employing the invention can also be used in chemical analysis apparatus of the kind described other than atomic absorption spectrophotometers.

I claim:

1. A burner for chemical apparatus comprising a burner plate having therein a flame aperture formed by a passage in the form of a slot, said passage extending through the upper and lower surfaces of said burner plate, the walls of said passage having a profile taken in a section located at right angles to the longer dimension of the slot, which tapers inwardly from a fuel inlet region adjacent the lower surface of said burner plate, said profile being at least partly arcuate about an axis parallel to the longer dimension of said slot.

2. A burner as claimed in claim 1, in which the edges of the slot in the lower surface of the burner plate have a said profile defined by the arc of a circle of radius 3 mm or greater. 

1. A burner for chemical apparatus comprising a burner plate having therein a flame aperture formed by a passage in the form of a slot, said passage extending through the upper and lower surfaces of said burner plate, the walls of said passage having a profile taken in a section located at right angles to the longer dimension of the slot, which tapers inwardly from a fuel inlet region adjacent the lower surface of said burner plate, said profile being at least partly arcuate about an axis parallel to the longer dimension of said slot.
 2. A burner as claimed in claim 1, in which the edges of the slot in the lower surface of the burner plate have a said profile defined by the arc of a circle of radius 3 mm or greater. 